This invention relates to devices for measuring particles suspended in a fluid and, more particularly, to forward light scattering photometers.
One type of device used for measuring the size and/or concentration of paticles suspended in a transparent fluid is photometer employing forward light scattering, i.e., a beam of light is projected through a flow stream of the fluid and the amount of light scattered out of the beam by the particles is measured by a light sensing or detecting means. Such photometers are used to monitor atmospheric pollution by smog, dust, pollen, etc., to monitor the presence of airborne pollutants of contaminants in an area where a high degree of cleanliness is required, such as laboratories, hospitals and rooms where dirt-sensitive equipment is operating or is being assembled, and to count the number of particles suspended in a liquid, such as cells in a blood sample or particles in hydraulic fluids.
To be most effective, such a photometer should be arranged so that only the light scattered by the particles in the sample being analyzed reaches the light detector. Typical prior art arrangements include a first lens for focusing an illuminating beam in a sensing region through which the sample is flowed, a second lens for focusing the scattered light onto the light detector and a circular light trap which is located between the sensing region and the second lens and is arranged to interrupt the unscattered portion of the illuminating beam and permit only the scattered light to pass through the second lens to the light detector.
The light scattering caused by the sample particles takes place primarily at small angles to the original path of the illuminating beam. To obtain maximum collection of the scattered light by the light detector, the light trap should be exactly the same size as the illuminating beam so that light rays scattered by only a slight deviation will pass the light trap and fall onto the second lens. If there are any spherical aberations, the illuminating beam cannot be perfectly focused on the light trap, i.e., the outer edge of the beam is not distinct. Consequently, the light trap usually must be somewhat larger than the illuminating beam so that the unscattered portions spilling over into the scattered light path do not travel to the light detector. As the overlapping margin of the light trap is increased, additional amounts of the scattered light are intercepted with resultant inaccurate measurements.
In order to minimize the overlapping margin of the light trap, prior art devices usually employ high quality lenses which will provide a very sharp image of the illuminating beam on the light trap and thereby minimize spillover of incident beams into the path of the scattered light. Such a lens typically employ doublets to correct for spherical aberations and are quite expensive.
When photometers are used for analyzing an aerosol sample, some means should be provided for shielding the lenses from the sample so that the particles from the sample will not deposit on the lenses. Any particles deposited on the lenses would produce inaccurate measurement by the light detector because they will cause a spurious scattering of the light.